Loudspeakers in vehicles pose special problems: typically the main loudspeakers are almost always, of necessity, installed in an offset location relative to the ears of the driver or passenger, consequently the high frequency response as perceived by the listener tends to be degraded unless specially located tweeters are deployed.
Due to the highly directional properties of high audio frequencies, and less-than-ideal mounting locations available for tweeters in vehicles, it is highly desireable to swivel-mount the two stereo tweeters so that they can be oriented and aimed to favor the listener's location.
In particular vehicles there may be flat panels suitably located for mounting tweeters so that a separate housing is not required; in some instances the flat panel location is such that no protrusion is allowable, thus flush mounting of the tweeter assembly is desireable.